In automotive technology, “brake by wire” brake systems are becoming increasingly common. Such brake systems often comprise a pedal-decoupling unit which is connected upstream of a brake master cylinder, whereby a brake pedal activation by the driver in “brake by wire” operating mode causes a separation of the direct hydraulic connection of the brake master cylinder with the wheel brakes and switches the hydraulic pressure output by the brake pressure-generating device to the wheel brakes. A segment of the action chain from detection of the driver's request to the build-up of a corresponding brake system pressure in the pressure-generating device is “by wire” and consequently not hydraulic. In “by wire” operating mode, the electrically controllable pressure-generating device applies brake pressure to the wheel brakes. To give the driver an acceptable pedal feel in “brake by wire” operating mode, “by wire” brake systems normally comprise a brake pedal feel simulation device. In these brake systems, the brakes can also be activated on the basis of electronic signals without the active involvement of the vehicle driver. These electronic signals can for example be emitted by an electronic stability program (“ESP”) or an automatic cruise control system (“ACC”).
If the “by wire” function is not ready for operation—for example due to possible failure of the electrically controllable pressure-generating device—the brake system switches to a fall-back mode which restores the conventional direct hydraulic coupling of the chambers of the tandem brake master cylinder with the wheel brakes. Thus a vehicle equipped with such a “by wire” brake system can also be braked on failure of the pressure-generating device, on failure of the electronic unit and even on failure of the electrical power supply, but then only with the setting energy provided by the driver on activation of the brake pedal.
The present invention is based on the object of fitting the fall-back operating mode of a “by wire” vehicle brake system with a brake force support which is independent of the function of the pressure-generating device.
This object is achieved according to the invention by a brake system according to this invention, wherein this comprises an electrically controllable auxiliary pressure-generating device by means of which the brake master cylinder can be activated, in particular by means of which a pressure in an intermediate chamber of the brake master cylinder can be controlled or regulated.
Further preferred embodiments or advantageous embodiments of the invention arise from the description below with reference to a FIGURE.
The invention is based on the consideration that an electro-hydraulic brake system which has a normal “brake by wire” operating mode and a hydraulic fall-back level must generate sufficient braking force in any operating state. This should be the case in particular when the control and regulating unit for controlling the pressure-generating device, or the pressure-generating device itself, has a malfunction or fails completely.
As is now known, this object can be achieved in that an independent auxiliary pressure-generating device is provided, by means of which the brake master cylinder can be externally activated to a certain extent. For this, the auxiliary pressure-generating device is connected hydraulically with an intermediate chamber of the brake master cylinder such that, on movement of a pressure medium into this intermediate chamber, a piston in the brake master cylinder is activated by the auxiliary pressure-generating device and hence pressure is built up in the chambers of the brake master cylinder and/or brake fluid can be displaced therefrom into the brake circuits.
The brake system in a so-called “brake by wire” operating mode can preferably be controlled both by the vehicle driver and independently of the vehicle driver; preferably it is operated in “brake by wire” mode and can be operated in at least one fall-back mode in which operation only by the vehicle driver is possible.
According to a first preferred embodiment example, the brake system for motor vehicles comprises a brake master cylinder which can be activated by means of a brake pedal, a wheel-brake-pressure modulation device to which the wheel brakes are connected, an electrically controllable pressure-generating device (preferably a cylinder-piston arrangement, the piston of which can be activated by an electromechanical actuator), a pressure-regulating valve arrangement to regulate and/or control a wheel brake pressure set at a wheel brake, and an electronic control and regulating unit (electronic unit ECU) which serves to control or regulate the brake system in a normal brake function without amplification by a servo (auxiliary pressure-generating device).
The pressure-regulating valve arrangement is preferably connected hydraulically to the brake master cylinder, the pressure-generating device and wheel brakes. By means of the pressure-regulating valve arrangement, the wheel brakes can be supplied with system brake pressure either by the brake master cylinder or by the pressure-generating device. The pressure-generating device activates the wheel brakes preferably directly via the pressure-regulating valve arrangement, e.g. the pressure chamber of the pressure-generating device can be connected with each of the wheel brakes via at least one valve.
Furthermore, the brake system according to the invention comprises an electrically controllable auxiliary pressure-generating device (an electro-hydraulic servo). This servo is preferably designed as an independent and also optional module of the brake system used in heavy vehicles, which could not achieve adequate braking effect without brake force amplification. In lighter vehicles and in vehicles with particularly efficient wheel brakes, this optional module can be omitted, whereby no substantial constructional changes are required to the remaining brake system (basic construction). This allows simplification of factory production of a brake system which is the same in principle for different vehicle types. This saves development and production costs.
According to the example, the servo or auxiliary pressure-generating device comprises an electric motor, a pump, a pressure sensor, a pedal travel or pedal angle sensor, an electrical energy accumulator, and a second electronic control and regulating unit or electronic unit. The servo preferably supplies an electronically regulated servo pressure which is passed via a hydraulic connection to an amplification-active surface in the master cylinder and there activates the master cylinder.
The second control and regulating unit or electronic unit of the servo (ECU2) is advantageously connected via a communication connection with the first control and regulating unit or electronic unit for normal function. The servo function is activated when necessary—for example when the first electronic unit for normal function is inactive and therefore does not respond to communication requests. The pedal position is determined and converted into a nominal servo pressure following a predetermined curve. This nominal pressure is then set by the second electronic unit of the servo by means of the electric motor, pump and pressure sensor.
Advantageously, the auxiliary pressure-generating device activates the brake master cylinder to set a nominal pressure, in particular a nominal pressure in the intermediate chamber of the brake master cylinder, when the first control and regulating unit and/or the pressure-generating device is partly or fully inactive or non-functioning. This means that the servo is used only when the regular pressure build-up by means of the pressure-generating device is not possible because of malfunctions and/or control faults. This ensures that sufficient brake pressure can be built up in the wheel brakes even on such a component failure. Such a situation can be detected via correspondingly selected signals over a communication connection described above between the first and second control and regulating unit.
Advantageously, the first electronic control and regulating unit is connected with a first sensor which determines the brake pedal travel or brake pedal angle or a value corresponding thereto, and the auxiliary brake pressure-generating device is connected with or comprises a second, in particular independently constructed sensor, in particular a brake pedal travel or brake pedal angle sensor, which determines the brake pedal travel or brake pedal angle or a value corresponding thereto.
The servo thus to a certain extent preferably comprises a pedal travel or pedal angle sensor to detect the pedal activation. A force sensor which detects the brake-pedal-actuating force would be too costly as a signal emitter for the servo, in particular in view of the modular design of the auxiliary brake pressure-generating device. By using a simple design of servo with no force sensor, the availability of the brake force support is extended to operating states in which the normal function of the brake system is not available.
Advantageously, using signals from the second sensor in the second electronic control and regulating unit of the auxiliary pressure-generating device, a nominal pressure is determined to activate the brake master cylinder. Signals from the first sensor are not required in this case. This embodiment supports a modular design of the auxiliary pressure-generating device.
In a normal “brake by wire” operating mode, a brake system pressure is generated preferably by the pressure-generating device. The pressure-generating device is advantageously formed by a cylinder-piston arrangement, the piston of which can be activated by an electromechanical actuator.
In a preferred embodiment of the brake system, connected to the brake master cylinder is a hydraulically activatable simulation device with at least one elastic element which, in particular in the “brake by wire” operating mode, gives the vehicle driver an acceptable pedal feel. In this operating mode the vehicle driver, on activation of the brake pedal, displaces pressure medium in the simulation device or simulator, wherein the pedal travel and/or pedal angle is determined and forms the basis for determination of the nominal braking moment. In this way, a reproducible relationship can be achieved between the pedal travel and a nominal value of the brake system pressure.
The advantages of the invention lie in particular in that with the auxiliary brake pressure-generating device, a measure for providing the brake system pressure independently of the pressure-generating device active in normal mode is achieved. In particular in a fall-back level, in this way the driver can be supported when braking the vehicle. With a modular design, the auxiliary pressure-generating device can be used in heavy vehicles in which an additional brake pressure build-up is required in order to fulfill legal requirements. In lighter vehicles this may be omitted. The other components of the brake system can then be used for both types of vehicle without adaptation. If a pedal travel or pedal angle sensor is used instead of a costly force sensor, the modular construction can be implemented systematically.